Does anyone know what these little brass weapon ornaments are actually used for?

I bought a couple of sets very much like these while shopping in downtown Los Angeles - the shafts are too thin for minifigs to grip, but after wrapping them with a carefully-measured amount of gaffers' tape they fit perfectly. (You can see a couple of them here and there in the BrikWars rulebook photos, but you have to look close.) They tend to bend and break apart if you play with them for very long, but for a little while they're the perfect non-purist minifig weapons.

Nice reference pic there, any idea how tall they are compared to a person? Some of those actually look like the new weird spear Lego has been producing, and others are easily made using existing pieces, even for purists.

...often feature these famous Chinese pole-arms, especially the kwan-dao.

Albeit the battle scenes in those movies often reflect the Chinese mythologies of super-warriors and levitation/flight ability... it's no different than the Western fantasy films like the recent "300", Eragon, Lord of the Rings, and etc. Albeit those Chinese movies have fiction in them, much like these Hollywood-made fantasy movies the weapons used are quite historical.

You can make it using the scimitar piece with either an antenna piece, 4L bar, or the 6L bar with stopper.

It is sometimes called Kwan Dao because its use was made popular by the famous ancient Chinese general Guan Yu (Mandarin pronunciation - Cantonese would be: Kwan Yu). But this type of weapon was already in use before his time.

I believe the proper name for this weapon is actually Qing Lung Yen Yue Dao (literally "Green Dragon Crescent Moon Blade"). The "Qing Lung" (i.e. green dragon) is just a prefix, "Yen Yue" refers to its shape (i.e. crescent moon).